In next-generation high-bandwidth low-power computing, the conventional on-chip copper interconnects, which route the electronic signals, will become bandwidth-limiting and also cause excessive electrical power consumption. Silicon photonics based optical interconnects are considered as one potential solution, where high bandwidth optical signals are routed through integrated low-loss optical waveguides. In contrast to the electrical interconnects (the copper wires), optical interconnects have many advantages, such as low crosstalk, immunity to electromagnetic interference, and low power consumption. Most importantly, optical interconnects are expected to provide a much larger bandwidth than the electrical interconnects. Wavelength-division multiplexing (WDM) technology provides a way to further increase the data capacity.
An optical light source is a key component for applications such as long-haul and data-center optical communication and on-chip optical interconnection. In the past ten years, silicon photonics has experienced unprecedented growth in research and development activities, as a possible technology towards next-generation multi-channel optical communications and optical interconnects for computer communication (compcom). However, silicon, as an indirect bandgap material, cannot emit light efficiently. While SOI (silicon-in-insulator) is a promising material platform for low-cost low-power-consumption devices for on-chip interconnection and optical communications, yet it lacks an efficient light source. This means that it is required to adopt an off-chip optical light source for signal transmission.
Although silicon is an indirect bandgap material which is not considered as an efficient light source, various solutions including erbium (Er)-doped silicon laser, hybrid-integrated evanescent lasers, and Ge-on-Si laser have been proposed, among which the hybrid-integrated evanescent lasers, such as a hybrid optical light source using III-V bonding to silicon wafer, are considered as the most promising way for on-chip light sources. Thanks to the enabling complementary Complementary Metal-Oxide-Semiconductor (CMOS) fabrication technologies, a myriad of essential silicon photonic passive and active components have been fabricated, including micrometer-scale optical filters, 10-40 Gbit/s-speed low-power-consumption modulators, and GHz-bandwidth germanium (Ge)-on-silicon (Si) and hybrid III-V-on-Si photodetectors.
Almost all of today's hybrid silicon (Si) lasers use a dual-material system, which means direct bonding of III-V onto a Si waveguide. The laser cavity is defined by the Si waveguide and the cleaved/polished interfaces, which means a relatively high cavity loss. The optical light is mostly confined in the Si waveguide, which means low amplification efficiency. The issues for such a dual-waveguide system for a laser cavity include: 1) The Si waveguide usually has ˜2 dB/cm propagation loss, which is relatively high, especially for some advanced light sources, such as mode-locked lasers where optical loss is critical; 2) The light is mostly confined inside the Si waveguide with only ˜4.3% light confined in the III-V gain medium, thus the amplification efficiency is low; 3) Cleaved/polished interface(s) for the formation of a laser cavity provides less optical control and low productivity.
Therefore, the problem for conventional hybrid Si lasers is the relatively high Si waveguide loss for the laser cavity, leading to a high threshold and high power consumption. Such high loss could be a major issue for advanced hybrid lasers, such as the mode-locked lasers.